I think there were a few flash cards around during the GB/C days, the Doctor Gameboy/Gameboy Doctor sounds familiar but they had very small internal memory, very high price tags and were no where near as user friendly as the GBA/DS equivilants. I do remember seeing someone mod a gameboy so it could load roms from an SD card which would give you a max of 64gb storage (bear in mind my complete romset is about 1 gigabyte) but there is no way they're being mass produced so at best you could persuade some clever hacker type to sell you a handmade one. Saying that someone could have made a more recent effort and there might be expandable memory flash cart out there somewhere. As Tsukurimashou said the flash cart linked to can only store the one game because apparently they were intended for devs to make games and not the mass piracy I'm used to. I wouldn't trust the product linked to in the OP since If I remember correctly they were plagued with graphical glitches and game ending bugs as well as the hardware itself breaking down far sooner than legal retail cartridges.